1. Field of the Invention
The present invention relates to an electric shutter adapted for use in a photographic camera and having a pair of solenoid devices for controlling the function of a shutter opening member and a shutter closing member.
2. Description of the Prior Art
Solenoid devices have beem employed in the exposure time control in the shutter since they facilitate the control of long exposure and enable automatic exposure control of a wide range in combination with a light-measuring circuit.
There are already known solenoids of various types such as a moving coil type in which a movable coil is moved by current supply with respect to a fixed permanent magnet; a current-release type in which an armature attracted by a permanent magnet-coil combination but biased to move away from said magnet is released from said magnet when said coil is powered to temporarily reduce the magnetic power of said magnet; a moving magnet type in which a movable permanent magnet is moved when a fixed coil is powered; and a current-attraction type in which a movable armature biased to move away from a solenoid is attracted thereto or released therefrom upon power supply thereto or termination of power supply.
Among these types, the solenoid device of moving coil type or moving magnet type can be used for the exposure time control by providing the moving member with a self-returning structure and so constructing said member to hold a shutter closing member when the selenoid device is not powered. In such case no particular means is required for resetting the moving member to the holding state after the shutter closing member is released.
However, such solenoid device requires a large electric power for rapidly releasing the holding state and becomes inevitably large in size.
Also the solenoid device of current-release type, having a permanent magnet and a solenoid in combination, is complicated in structure and the coil tends to become large since it has to cancel the magnetic power of the permanent magnet. Also a resetting function is needed in order to return the moving member from a position for releasing the shutter closing member to the holding position. On the other hand, the solenoid device of current-attraction type, though requiring a resetting function, is capable of attracting the armature with a low current, since the energization of the coil can be initiated in a state in which the armature is maintained in contact with a yoke.
For a given current consumption, therefore, the solenoid device of current-attraction type is evidently most suitable for size reduction.
In order to use such solenoid device for controlling the shutter opening or closing member, the solenoid device is so constructed that the moving member prior to movement holds the shutter opening member or shutter closing member but releases said opening or closing member upon movement of the moving member, thereby initiating the shutter opening or closing operation.
Also there is known an electric shutter having two solenoid devices respectively for controlling the function of the shutter opening member and the shutter closing member. For the purpose of clarity, such electric shutter shall be hereinafter called two-solenoid type electric shutter. The use of current-attraction solenoids is desirable for compactizing such two-solenoid type electric shutter. In such case, however, there will be required a pair of resetting devices for the resetting operation, and there will also arise a problem of mechanical linkage of the shutter mechanism, including the resetting devices, with other mechanisms.